FIG. 4 shows a prior art automatic gain control system used in a spread spectrum receiver. In this drawings, reference numeral 1 refers to a correlator, 2 to an IF (intermediate frequency) amplifier, 3 to a correlation detector, and 4 to an AGC (automatic gain control) amplifier.
The correlator 1 is supplied with a received spread-spectrum signal and gives a correlation output to the correlation detector 3 via the IF amplifier 2.
The correlation detector 3 produces an output which exhibits a correlation spike A as shown in FIG. 5. When the correlation spike A is large, the AGC amplifier 4 exhibits a large output level and controls the IF amplifier 2 to decrease its gain.
When the correlation spike A is small, the AGC amplifier 4 exhibits a small output level and controls the IF amplifier 2 to increase its gain.
As described, the prior art automatic gain control system uses the correlation detector for detecting an output of the IF amplifier. Therefore, when two convolvers are used as the correlator 1, two correlation detectors are required to detect respective correlation outputs from two convolvers, and this invites a complicated circuit arrangement and an increase in the manufacturing cost.
The prior art system also requires two IF amplifiers and corresponding two AGC amplifiers, and this further complicates the circuit arrangement.
Beside this, in case of a waveform such as the correlation spike A, an AGC circuit which follows the peak value is used generally. Such an AGC circuit is provided with a short time constant for electric charge and a long time constant for electric discharge. In this case, however, the AGC circuit, although quickly responsive to a change causing an increase of the correlation spike, exhibits a slow response to a change causing a decrease. Further, although the discharge time constant is long, the AGC circuit discharges little by little continuously when the correlation spike does not exist. Therefore, the AGC control voltage changes continuously, and this invites an erroneous operation caused by a noise or spurious response during absence of the correlation spike.
In an SSC receiver, direct detection of an SSC signal is impossible. Therefore, a carrier sense signal indicative of whether the SSC signal is received or not is obtained using a correlator output. The above-indicated conventional carrier sense system using the AGC system, as shown in FIG. 16A, detects a correlation spike S.sub.1 of a correlator output by comparing it with a threshold value S.sub.2, counts the number of detected correlation spikes S.sub.3 in a counter, and outputs a carrier sense signal S.sub.4.
In this system, however, when the received signal level is weak, as shown in FIG. 16B, the number S'.sub.3 of noises N detected by an erroneous operation of the correlator caused by the noises is accumulated, and an erroneous carrier sense signal S'.sub.4 is produced.